there are some subtle autocompletion problems in bash 4.4.18 in latest rc1 from 18.06:
-. - shortcut for latest argument doesn't work as expected. It should cycle in the last 'n' arguments and it does repeat latest argument
double tab doesn't produce possible-completions ( although -? produce it as expected ).
There can be possibly more problems, but I report the ones I tested and know how they should work.
I'm not certain of what issue you're having, it was noted that Busybox is used by default. In addition, history is not compiled into the OpenWrt version of Bash.
This was noted at Post No. 18 on the link I provided above:
If your issue is with Bash, you likely have to compile a custom Bash with the features you desire.