My stations are configured as follows:
https://snipboard.io/UmEQgh.jpg
https://snipboard.io/8LvlBO.jpg

In this case, there are 4 tanks of 25,000 (100,000 oil in total)

I expect this station to only request oil, but LTN adds it as a supply station. Is this a bug or am I doing something wrong?
I tested this on versions 2.4.2 and 2.3.4

you have more than the theshold, than it is going to be a provider.
you should limit the load behavior - you have loaded the trains more than requested, so the station should be at max what you request . 50k to your screenshots.
But you could set also the "provide threshold" to x millions,

Yeah, all stations are actually both request and supply stations. To enforce requesting-only, set the supply threshold to be very high. In the example you gave, you set the supply threshold to be 50K. If you had set it to 50M, you would be fine.

This is done automatically for you if you use LTN Combinators.