I also just tried 1.5 and the Egyptian one gives the same results.
I also tried updating to 1.5, deleting the mashup pack from storage and redownloading it.
I'm thinking about contacting Nintendo for a refund at this point on the dlc. I understand that fixes aren't immediate but it feels like the 20 I spent was a waste and Mojang doesn't seem to be addressing it.
Not sure if this helps research but I was playing around last night and found that if I made an Egyptian template world in Switch Edition and then converted it into the Bedrock version, it does work with no error messages or missing textures.
I tried deleting everything from both versions of the game since I didn't really have any worthwhile worlds bc and now it doesn't even think I own my dlc. Am I going to have to download the old version again just to prove what I own?
I can confirm it worked for Adventure Time and Egyptian and didn't break Fallout which is still working. I did download them all fresh.