This may be related to the post at https://forums.developer.amazon.com/questions/209498/alexapresentationaplexecutecommands-command-stoppe-1.html.
I have two skills on the development platform that use APL and SpeakItem. I had not used them in a while and when trying them out, one had terrible formatting and no highlighted speech though it does speak. The other simply did not highlight the speech but it too speaks. I tested on both an Echo Show V1 and Echo Show V2 (firmware 6415749120) with the same results. The code had not changed in months. The skill with bad formatting still looks fine when using test console in the Alexa Developer Console.
On Prime Day I ordered three Echo Show 5's and I received them yesterday. I setup up one of them, it downloaded firmware updates (0002584226692 - whew!) and was ready to use. Just for the heck of it, I tested out the two APL skills and they worked fine! I was shocked. I tried them again a few minutes later and they looked great. I assumed that the newest firmware probably fixed something and that it would just be a while before the older devices caught up.
An hour or so passed and i wanted to show someone the skills working properly. I ran them on the Echo Show 5 and they behaved just like the older devices. They no longer rendered my skills correctly. So last night I set up another Echo Show 5 and did the same thing. It worked great with the two APL skills and there were no display issues. An hour later I went back and tested the skills and they are not working correctly. I still have a third Echo Show 5 to set up but I assume it will do the exact same thing.
Obviously there is some kind of update running in the background after the device is booted and running that is killing APL SpeakItem functionality and also affecting certain formatting. I found other posts regarding similar problems but nothing indicating it starting to happen on a new device after it has been running a while. I am hoping this is just a "Beta" issue with APL and that there is a fix in the works. I have seen several issues reported going back to May so I will cross my fingers.
UPDATE: I did some further testing. I powered down the Echo Show 5, pulled the plug and then powered it back up. No change. Then I did a factory reset and tested it. The APL skills worked again! I did periodic testing and sometime after my device was running for about 1 1/2 hours, the skills again stopped working properly. Definitely something going on in the background that is causing the problem.