Is it normal for the liner to just sit loose at the walls after a crawl encapsulation?

You're not overthinking it. That edge is supposed to be sealed and fastened, and what you're describing is a lazy finish. On a proper job the liner either runs up the wall and gets attached with a termination bar (a strip of metal or plastic screwed through the liner into the block, sealed with caulk or tape over the top), or it's sealed directly to the cured foam with foil or poly tape and mastic. Just letting it flop against the wall with nothing holding it is not how it's done, and the weight of the liner definitly isn't doing anything. Those liners are stiff, they curl, and they pull away from walls over time. Give it a season of humidity swings and you'll have gaps you coud poke a finger through, which is exactly what you can allready do.

Been doing crawl spaces since back when we just threw down 6 mil poly and held it down with old shingles, so I've seen every shortcut there is. The bare band for the termite guy is standard, my own inspector makes us leave that gap every time, so no worries there. But before the dehumidifier guy comes, check whether they sealed the seams in the liner itself and around any piers where it runs up and over. A dehumidifier will pull a vacuum on that space and any unsealed edge will quietly feed it humid air all summer. I'd also ask them what foam thickness they sprayed, because a couple inches on block walls in the south is usually doing most of the work against condensation.

One thing I'd add from my own crawl, where I had almost the identical unsealed edge at the block: the real tell is what shows up on the wood, not the liner itself. Take a flashlight and look at the sill plate and the bottom few inches of the joists directly above that band of bare foundation wall. Mine had faint shadowing and a musty smell concentrated right at that line, which is how I figured out soil air was rolling up behind the foam all along. If yours is clean and dry up there, you caught it early, but I'd still make them come back before the dehumidifier goes in, becasue once the space is sealed tight and pressurized differently, any leak path that exists is the one the air will use.

FWIW I'd call the company now rather than wait. When I let a similar sloppy edge slide on my own job, the warranty talk got way harder once a year had passed. Take pics with a tape measure or somthing in frame for scale, email them, and get it on record. Also when the dehumidifier guy shows up, ask where the condensate line's draining. Mine dumped right against the block and I had to reroute it. If the liner edge gets fixed before that visit, they can run the line cleanly without tripping over the repair.

Update: called the company the same day and emailed pics like the last reply suggested. Turned out the crew skipped the final step, oops on thier end. They came back and fastened the liner up the wall with a termination bar and caulked the seam. Checked the sill plate with a flashlight while I was down there too, all clean. Feeling alot better before the dehumidifier goes in.