Strange reboot on 2927

I've not seen this before and I wonder if it's a "feature" of firmware 4.5.2.1

I found that my 2927AX had rebooted while I was out yesterday.

From the logs and other monitoring it seems related to a process related to Draytek's DDNS

acme client: certificate operation: Update
acme client: certificate expire time within 30days, start renew
acme client: looking up for let's encrypt server ...
[LICENSE][HTTPs][DDNS get license ok]
acme client: start dns challenge
acme client: Update acme token successfully.
acme client: Error: server response pending: 0
acme client: waiting server to verify domain .. 
acme client: domain verify success

At that point the router seems to have rebooted.

The router came up correctly and all seems well afterwards but I don't recall a random reboot like this before.

Is this expected as part of a DDNS certificate renewal process?