As the title says. Im using the Regeneration-1.14.4-2.0.9-SERVER-FIX version without any other mods. On the image below you can see an example of this. Even if I manage to stack them, after closing and reopening inventory, they will be separated.

Does this happen on a new world without regeneration installed?

I created a new world without the mod and the issue is gone.

Ah I see, could you show me the tooltips for both stacks

Here you go

Strange! I'll investigate it for you!

I don't know if this might help but I remember this issue occuring in Regeneration-1.14.4-2.0.0 and was later gone in Regeneration-1.14.4-2.0.1

yes for me it is

Is this issue still a problem?

I checked why it doesn't stack using NBTExplorer. In inventory items just crafted or taken from creative inventory has 4 entries: -tag (with 0 entries), count, id and slot. Items placed and collected has no tag entry, just count, id and slot. This tag makes items not stackable.
For now there are 2 ways to stack it enyway.
1: Place and destoy all just crafted or creative items. Works fine with torches. With that method placed and collected item will stack corectly. Harder but make life easier after.
2: Throw items you want to join (like planks shown before) out from inventory into same place. After moment items will join. Good if you just want save some space in inventory, but placing and destroing this items make it unstackable again. In that case you need reapeat 1st or 2nd method or ignore, but this is so irrytating, that makes it almoust imposible to ignore.