This is sort of not possible to fix without one of the options:
Adding an "italics":false to these item's display.Name, and adding a special NBT tag to items to prevent item names from showing in frames (display.ShowNameFramed?)
Add another item type for these items, which is backwards incompatible.
Added.
@Galaxy_2Alex Note: I am able to reproduce this with Vanilla 1.16.2 Singleplayer.
This is not relevant to the bugs section.
The game has been overloaded (guess from your tick speed issue, and the frozen entities), which has caused it to have slow saving.