Hello Milbo,
Thanks for your comments.
>> ... 12.50 x 10 = 124.50
> looks like rounded to 2 decimals. And I have no clue about your other settings, which you are maybe not aware of.
This: >>"Clean new install of Joomla 3.8.6, VM 3.2.12.9708 / I deactivated all rules (tax, discount) and rounded to 1 decimal."
This is literally all I have done and set (as far as I remember, using default demo data). Single currency. I have (on my shop under development) tried all settings I could think of and that I have found mentioned in the forum and documentation, the results are always the same. "Rappenrundung", "Round on display" ... no difference. VAT is not relevant for the problem.
>> What I also found is that it is all good with the default currency
I was wrong about that. I only did not see the issue, because the prices in the default currency are already rounded to 1 decimal, so additional rounding makes no difference - unless you add discounts and/or VAT.
> Why you cannot show it on the product page?
Because it is wrong. 12.50 x 10 = 125.00, not 124.50. Customers don't care about the mathematics behind these numbers, they will just be confused. Most won't notice or won't care. But my client noticed and cares.
I have looked through the source code, but couldn't figure it out myself, unfortunately. I don't see why the total price cannot be calculated by rounded base price times number of units rather than however it is done currently.
If only the base price is affected by any modifications and rounded afterwards, the total will then always be correct automatically. Whatever is set in the backend, numbers should always add up in the frontend, IMHO.