I was finally able to reproduce the missing IPs. It's indeed related to the Linux kernel not having the device's MAC in its ARP table - it can take a while for this to happen, and it depends on the clients.
I was also able to reproduce the exact same issue with 380.61. This is NOT new to 380.62.
Asuswrt-Merlin relies on the ARP cache to determine the IP of a given MAC address. Now, when a client connects to the AP, it does not communicate with that AP at an IP level, but only with the main router. That causes IPs to either quickly disappear from the AP's tables, or flat out never appear in them.
The only way for an AP to be able to know about the IPs would be to scan the entire subnet, causing the AP cache to get populated. This happens for instance when networkmap scans the LAN to build a list of clients. You can see this by accessing the networkmap (first page when you log into the AP), wait 2 minutes, then go back to the Wireless Log page. Any missing IPs should now be visible... for the time being. That is, until the kernel flushes its ARP tables of these stale entries.
An alternative might be to rely on the networkmap database instead of the arp table to retrieve IPs. However I'm not really a fan of that idea, as its has its own drawbacks (stale entries, or missing newly connected clients).
So in summary:
1) It's nothing new, it was also happening in 380.61
2) It's related to the kernel's handling of the ARP table, forcing a refresh would have significant side-effects (like waking up any device in sleep mode on your LAN)
Best I could do would be to see if I could rely on the networkmap database as an alternative IP source (i.e. ONLY when the ARP table doesn't have an entry for a client). I will have to see how tricky this would be to implement however. The networkmap database is often a bit unreliable, with its own issues...