When the LTN combinator is placed because of upgrading a normal combinator, it takes whatever provide setting was on the original combinator. However, as a requester station, it is then impossible to make it have a high provide value, even though that option is enabled in the settings.

I understand not really wanting to modify existing settings, but having it like this is also not a good solution.

Yeah, this causes problems where your trains will try to pick up resources from request stations, halting everything until you notice and go over to try to fix the issue by reducing how much is in the inventories, or adjusting the request values.

The only thing that completely prevents the issue is to set the request threshold to 0 so you don't go over the request amount. This will create disruption as you'll be out of the item until a train can restock (maybe have some extra buffer not connected to the requester network).

maybe add an option for a provide threshold, or have a provide threshold automatically set to a max value when in Request mode (and vice versa).

There is an option for a provide threshold. It's in the settings, and defaults to on. If you build a LTN combinator from scratch, it works fine. If you upgrade an existing combinator to an LTN one, then it doesn't set this value if you didn't have it set before

I figured out that making that station a Provider for a second and back, will re-set the provide threshold to the high value (50M by default)