I would ask your CRS if they think you may be taking too much miralax.
Sometimes if the BM is soft and not urgent it doesn't stimulate the rectum and sphincter enough to fully complete the BM so you end up needing to go again a few hours later. That could be what you are experiencing, it's hard getting a BM the consistency that works for our bodies I need mine to be a Bristol 3 others need a Bristol 5.
I'm sure you will find what works best for you eventually