I have a document with e.g. 70 DPI, some text inside has e.g. 12 pt font and I when change the DPI of the document to anything else, let's say 300, then all fonts of the text shrink to 1.8 pt or something like that. I can't even copy the text to another document and then back. They still have broken size.
I think font size should be preserved with a change of a document DPI because I need to export the same document with different DPIs (e.g. for print 300, for web 70).
The only workaround I found is to copy all texts to Apple Pages, change document DPI and them copy & paste texts back into Affinity Design.