the other tiny trees grown into different logs, but the azalea seems to be missing theirs and instead is replaced with oak

Its not a bug but a feature

This report is invalid, as this appears to be a feature request. Please do not post suggestions on the bug tracker as this place is specifically used for tracking and handling bugs only. If you wish to share your ideas, you can do so on some of the following links.

Official Minecraft Feedback Page - https://feedback.minecraft.net/hc/en-us
Minecraft Suggestions Reddit - https://www.reddit.com/r/minecraftsuggestions