My little girl did/does this too. Certain foods seem worse like toast or anything with a bit of a skin like green beans. I think the roof of babies mouths are deeper than adults so food accumulates up there? And maybe they’ve not got the tongue strength or control yet to move it down? It’s happening less now she’s 10 months, but happened a lot since starting solids.
My son did this for ages! Kept a bit of broccoli for two hours once! I found giving him a full sippy cup of water when I noticed he was holding food helped to wash it down or out. He didn’t drink the whole thing but I always make sure he has lots of water with each meal. He loves a sippy cup and taking the sips himself seemed to help. He would then go in to continue eating or sometimes he would hold food when he was full. He’s now stopped doing it I think and he’s 9.5 months.
I've seen a few posts like this on the BLW Facebook groups. Apparently, it's completely normal for babies to burrow food, kind of like a hamster. They recommend offering water to help them wash it away, and to absolutely never put your fingers in their mouth to try and get the food out as you're at risk of pushing it further down.