Ah, I see the problem with the file append... you were trying to create a new file using append, and Classic99 didn't pass back correctly that the file didn't exist. That was an important datapoint. The file didn't exist, so the program for some reason fell into the "PC file" mode and considered it DF128.
Apparently you're either the first person to create a file in Classic99 with APPEND or nobody has ever told me.
That happened to me when I was working on Forth words to append Forth code to a file. It failed if the file did not exist. I did not test it on real iron—just assumed that opening a non-existent file in APPEND mode was illegal since opening such a file in OUTPUT mode is essentially the same as APPEND mode. I will try to find the thread that demonstrates this.