It's a glitch that is actually sort of working with the way Covid is rolling. If you want to make it happen permanently though, you've got to move all of those players off of their CHL teams, and onto the AHL teams before starting your franchise.
My understanding is that any player, under the age of 20, who's CHL rights are held by a team, can only play in the NHL or CHL. Once the CHL teams season ends, the player can be recalled to the NHL or AHL. If they're drafted from Europe (Ie. Rasmus Sandin), they're able to play in the AHL as 18 year olds.
The actual agreement is here, but very wordy:
https://olis.leg.state.or.us/liz/201...ocument/143414