FCP’s Flicker filter is for interlace flicker, not fluorescent light flicker.
You might want to check out Foundry’s Furnace plugins for FCP. They have a flickering light filter which is reputed to be fairly good. Go to toolfarm.com and look it up. It’s on sale right now, so you’re in luck!
Arnie
Post production is not an afterthought!
https://www.arniepix.com/