After creating a new character and accessing bank it's size has reset to 10

What the title says. I created a new character (Fl4k, the other one is a lvl 50 Amara) and played him up to Sanctuary. I then accessed the lost loot machine, sold some of the loot and bought back pack SDU’s. Proceeded to access the bank and picked up 10 items from it. The bank said it was 40/10 full, because, obviously, it’s a shared bank and I had bough all SDU upgrades and filled it with my Amara.

Finally I had some space in my bank so I switched to Amara and fast traveled to Sanctuary to unload some of the 40 legendaries she had in her backpack. I tried to store items but was unable to and then noticed it said that there are 40/10 items in the bank. Wtf I thought and went to see Marcus and bam, he had a level 1 bank SDU for sale. I had lost all my bank SDU’s. This can’t be done on purpose right? I get it that you need to buy bank SDU’s for all your characters to be able to store up to 50 items to the bank, but accessing the bank shouldn’t remove existing SDU’s from other characters? Right, riiight?

That is correct. It sounds like your Amara 50slot bank file got deleted, which in the past would have given you an empty 10slot bank. Instead you got all 40 items shoved back into an empty bank.

The bank also holds your trinkets, skins and Eridium. <- have you checked those?