I have been in situations where 1 segment of my domestic F award was in XN, and there was F availability for upgrade prior to boarding. However, since the award was issued with the segment in XN, I was not able to get the F seat. Both gate agent and 1K agent weren't able to help.

You're lucky to have been able to get the 2 F seats.

I also had international F award TG F BKK-FRA, then LH C FRA-IAD. When we landed in FRA, I checked and saw LH F space availability for UA award. I called 1K desk and had the tickets reissued to LH F.

Your issue is what is considered a reasonable frequency of inventory check. You're the only one who can answer that question. If you succeed as in this case, then it was worth it.