After upgrading to 25.1 GPU previewing is erratic. After a while working on a document GPU previewing is not available (after restart Illustrator it is). Then also problems with text selection occur, text is not marked as selected and text cursor is not visible. Seems like a problem with out of video memory? Using a maxed out MacMini with 64GB RAM and a Blackmagic eGPU. MacOS is 11.1
For now reverting Illustrator 25.0 removes all problems.