When installing Pipe Plus, my configured underground pipes revert to the defaults. Can Pipe Plus respect the configured values and not revert to the default 9 tiles ?

Configurable Underground Lengths: https://mods.factorio.com/mod/ConfigUnderLengths

This should be no problem.

Edit: Done.

Nice, it works !! Thank you !

I'm still having this problem. I'm using Configurable Underground Lengths and this mods seems to set underground pipe lengths to my limit - 1. It effectively disconnects a majority of the underground pipes in my existing base.

The mod doesn't change anything but reads the settings from the Configurable Underground Lengths mod (the first one to be specific).
I may have to set it as optional dependency too. I'll look into this.

You're doing your changes in data while Configurable Underground Lengths in data-final-fixes. I think that mod grabs the distance from your mod. I can't be arsed to go through the full Lua code Configurable Underground Lengths since it uses shortened variable names which are hard to read, but I did find this:

local skip_dist_old = con.max_underground_distance - 1

This is in the method which generates underground pipe prototypes to change the distance of. It seems like the mod expects only one pipe-to-ground item per "tier" and is grabbing yours then reducing it by one. You already have a special-case exception for when that mod is active. You could simply increase the fallback underground distance in that case by one. I tried it myself, and it retained pipe distances as I've set them:

if mods["ConfigUnderLengths"] then
dist = tonumber(settings.startup["config-under-lengths-pipes-1"].value) + 1
end

I haven't tested this extensively, but it fixed my issue.

I've set the distance to 24 and tested every underground pipe. All underground distances have the same length but the CUL mod sets every distance to 23 instead, so reducing it by 1. The -1 you mentioned i think.
froom what i saw he checks for the -1 to see if there is a change set and adds 1 otherwise. He then must substract the value at some point again to get whatever the user had set initially.
Im not shure why the author did this and it's a bit late to take a closer look but i will see through it tomorrow.

It seems to be that there is a check for pipe_connections[2] which requires other mods to have their tables in a specific order. The second thing, and this caused the length differences, is that he adds +1 to whatever the user enters as distance which makes some sort of sense since codewise the underground distance is 10 tiles long. Factorio just uses the last tile as the other end and so we get 9 tiles in between both sides.
A bit weird. So i need to do the same and now, everything should work as expected.

That makes sense. Glad you pieced it together, because I kind of got lost in the CUL code. Tried it, can confirm it works in at least the cases I've tried. Thank you kindly.