Hi. I'm facing the same issue. My Compressor version is 4.4 and the current App Store version is 4.4.6 (Catalina). The update doesn't work. I switched recently from Sierra to High Sierra and could update FCP. App Store proposed me not the last version 10.4.8 compatible with Catalina but an older one (10.4.6).
Even if the Compressor version 4.4.2 should be compatible with High Sierra, I wonder if my MBP (early 2011) allows this.
Any idea ?