BKK-MIA award routing (award search issues)

Hi guys,
New to UA and trying to figure out what I'm doing wrong.

Try to book BKK-FRA-IAH-MIA as an award.

For the dates I'm looking at, United shows availability for each individual segment at the Saver level. I can also book BKK-IAH (first two segments) and FRA-MIA (last two segments) as legal Saver awards, so connection times shouldn't be the issue.

However, when I try to get BKK-MIA (either through a one-way search or even a multi-city search spelling out each segment), the award search errors out.

How do I fix this?

Tried search and saw some similar things but no super-thread - if I missed something feel free to move this.

Thanks!