You may want to experiment with the options at Tools | Options... | LibreOffice Writer | Table | Input in tables. I don't claim to be an expert in exactly what these do, so I won't attempt to explain.
But there is one thing of which you should be aware. In some circumstances, any input that does not fit the applied format - in your case, Currency - will automatically reset the format of that cell alone to Text. Note that something as simple as completing an entry into a cell with Enter (instead of an arrow key or a mouse click) will render your entry no longer acceptable as Currency and have this effect. Interestingly, if you press Enter and then Backspace to correct yourself, the problem persists, but Enter followed by Edit | Undo (or Ctrl+Z) works without resetting the cell format. Note also that if you paste in a number from elsewhere, it is quite easy to bring along a trailing paragraph break, just as if you had pressed Enter, and this will likewise cause your problem
When you say that you "can't force it to apply currency numeric format", do you mean that reapplying the format has no effect? The only reason I can imagine for that would be that your entry cannot be interpreted as Currency: perhaps it contains a rogue character or has misplaced thousands separators, for example.
I trust this helps.
Brian Barker