max-ixfr-ratio appears to be forcing AXFR very prematurely on BIND 9.16.15
We have three reports of problems with authoritative servers frequently (most of the time) responding with AXFR instead of the requested IXFR since updating to BIND 9.16.15. In two of the cases (we have not yet received results from the third), disabling max-ixfr-ratio returned the servers to their expected behaviour:
max-ixfr-ratio unlimited;
The symptom appears to be a problem in the calculation of the comparative sizes of zone and incremental update - although it may turn out to be an issue with .jnl file format (each increment header carries the 'size' of the update), or with the the way that the size of the zone itself is determined. In at least one case, the zone involved is very large.