I had a bunch of inserts like that on my Chamber Pro's when I took them apart. Since the bolt was showing thru before I took it apart, I didn't see it as an issue and not causing leaks. Based on the new inserts I bought, I figured there was either some machining in the threads that allowed the bolts to push them down like that. I've had to use a tap to chase some of the insert threads before putting them in.
If those are like what I dealt with, the red loctite they use on the inserts makes them a bear to take out. I used a heat gun to heat it up then sprayed them with WD40 penetrant. On those that had a lot of corrosion and didn't break free right away, adding hydrogen peroxide loosened them up (I put a cork from the bottom to hold the fluid in the threads to let it work better.
I bought a cheap long handle wide screwdriver and ground it to fit without touching the threads on the wheel. make sure you weaken the loctite before trying to back out/push thru the inserts, I broke the tabs on the insert tool trying to remove the wheel level inserts before weakening the loctite.
Once you get them out you can use a tap to clean up the threads, measure the the outer insert threads for the proper tap.
I bought 2 taps, one for the outer insert threads and one for the inner (bolt) threads. it's been a royal PIA but dealing with this many corroded bolts has been a bigger PIA.
Texas humidity is corrosion hell for dissimilar metals, next time I put them together I'm using anti-seize on the bolts.
long winded I know lol