Chances are you are going too slow and there is something mechanical going on.
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mechanical: as @scottybeefs pointed out, even with suboptimal feeds and speeds, you should not get a wavy edge / irregular circle like that. When you mentioned the same job works fine in bamboo/soft wood, did you retry in wood recently ? It may just be a loose V-wheel that used to be tight. It’s worth a check anyway, at least grab the router collet with the machine powered on (and the router unplugged) and see if you can detect any slop when trying to move it, and/or check whether any V-wheel can spin freely.
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feeds and speeds: soft plastics (e.g. HDPE/Delrin) like fast feedrates, and the answer is very often “go faster”. In this thread we discussed/experimented vcarving in HDPE and the setting I tried back then was a 60° vbit, 10.000RPM, 60ipm. Since you are using 20500RPM, the equivalent feedrate would be ~120ipm, so basically double of what you used.
Also, make sure your vbit is still sharp. Plastics need sharp cutting edges, so vbits that have seen a lot of abuse in wood may perform poorly in soft plastics.