Timing Issues with Optically Isolated Data Signal
Looking at the ACPL-247 datasheet, the off time is typically 40us. So the isolator takes 40us typically to turn off, in your setup it is faster and only taking 25us to trip the BJT.
Note you do not see it on the on time because it is specified 2us, although at 200khz I would think it would show up... So you have a 200khz waveform?

Try another isolator, I'm 99% confident that a better one will solve your problem. If you really need 200khz, consider something with galvanic isolation or an isolator designed for digital logic signals.

See the screen capture below for datasheet summary...