I'm on 18.2 (22C152), and I think the problem started with 18.2. I haven't reset the keyboard dictionary (I don't want to lose my text replacements), but I too removed the Emoji keyboard and did a hard reset several times, but still the frequently used stickers doesn't reflect the ones I've used recently. Stickers from some sticker packs do show up in the frequently used area, but about 2/3s of the sticker packs' used stickers do not show up there. This is a hassle since for stickers, I often text them to myself, specifically so they show up in the frequently used area so I can find them quickly when I send the to other people.