looks like horizontal 1xN chest where N<10 are not created. (vertical are ok)
Merging tool are not working for them and the chest are not in the list of entities in the Map Editor.

I use unlimited chests, settings max: 55x55x100
filter: 1xN Nx1 2x2 2x3 2x4 2x5 2x6 2x7 2x8 2x9 2x10 3x2 4x2 5x2 6x2 7x2 8x2 9x2 10x2 3x3 3x4 3x5 3x6 3x7 3x8 3x9 3x10 4x3 5x3 6x3 7x3 8x3 9x3 10x3 4x4 4x5 4x6 4x7 4x8 4x9 4x10 5x4 6x4 7x4 8x4 9x4 10x4 5x5 6x6 7x7 8x8 9x9 10x10

Also,

• does the "hard limit" (not the max settings) affect performance?
  you could set it to 55 instead of 42, for trains

• I understand that from internal point of view 2x5 and 5x2 are different chests,
  but in the whitelist they both could be enabled by any of 2x5 or 5x2, so whitelist could be 2 times smaller.
  And maybe checkbox, what can return strict behaviour if disabled: "rotation indifference"

I can't reproduce the issue with this setting. Are you trying to merge a chest which isn't mergable? Does it show as selected in the selection during merging?

• Increasing the limits creates more entities in the game. There is a minimal performance hit because of more memory being used for storing the entity prototypes. There shouldn't be a UPS problem as long as you have memory to spare.
• Good idea, will implement in the future

yes, I select chests (for example 6 in a row), they glow yellow as usual, but have not merged when I release the mouse button.

I tested it with the only editor extentions, merged chest and merged chest unlimited.

I tested with different settings, there is no problem with default settings and whitelist 1xN Nx1,
but the problem appears with the whitelist:

1xN Nx1 2x2 2x3 2x4 2x5 2x6 2x7 2x8 2x9 2x10 3x2 4x2 5x2 6x2 7x2 8x2 9x2 10x2 3x3 3x4 3x5 3x6 3x7 3x8 3x9 3x10 4x3 5x3 6x3 7x3 8x3 9x3 10x3 4x4 4x5 4x6 4x7 4x8 4x9 4x10 5x4 6x4 7x4 8x4 9x4 10x4 5x5 6x6 7x7 8x8 9x9 10x10

In the log I have multiple
1.527 Script @WideChests/data_init.lua:109: Chest steel-chest nil

and the checksum for the unlimited is 0:

1.647 Checksum for core: 905122912
1.647 Checksum of base: 1054619811
1.647 Checksum of WideChestsUnlimited: 0
1.647 Checksum of flib: 574468295
1.647 Checksum of EditorExtensions: 1353132028
1.647 Checksum of WideChests: 245647953

the log:
https://justpaste.it/evnkl

In the map editor, the first 9 chest are all vertical, the 11th chest is the warehouse, and no horizontal,
but with the default config there are 9 vertical and 9 horizontal before the warehouses:

https://i.imgur.com/Bn3AJNY.jpeg

UPD.
6-wide chest is created with the
1xN Nx1 2x2 2x3 2x4 2x5 2x6 2x7 2x8 2x9 2x10

6-wide chest is not created with the
1xN Nx1 2x2 2x3 2x4 2x5 2x6 2x7 2x8 2x9 2x10 3x2 4x2 5x2 6x2 7x2 8x2 9x2 10x2

In the log I have multiple
1.527 Script @WideChests/data_init.lua:109: Chest steel-chest nil

Lol, that's just a debug print I forgot to remove

work with the
1xN Nx1 2x2 2x3 2x4 2x5 2x6 2x7 2x8 2x9 2x10

doesn't work with the
1xN Nx1 2x2 2x3 2x4 2x5 2x6 2x7 2x8 2x9 2x10 3x2 4x2 5x2 6x2 7x2 8x2 9x2 10x2

Ok, will try this again. I guess you are trying to merge 6 wide steel chest? And could you send me a screenshot with your entire mod settings?

there it is, only whitelist is changed:

https://i.imgur.com/l3GEAc2.jpeg

I am trying to merge 6 wide steel chest

Ok, was able to reproduce

I have a similar problem here. I tried to split the merged boxes before the update and then merge them again.

Interestingly, horizontal boxes do not work at height 1 with an exact length of 12 or 16 (which was connected before the update). Shorter and longer boxes work as far as I've tested.

I use SE +K2, as well as Merging Chests unlimited.
My number of unmerged boxes is approximately 51k, maximum box width and height 500, maximum area 500, whitelist configuration: 1xN Nx1 and 25 detailed configurations, the largest of which is 18x5.

I hope my observations have been helpful and, with best regards, I remain on version 4.2.0 for the time being.

Thank you for this great mod. Don't let it throw you off track when things get bumpy.

Fixed in 5.0.1