Thanks for your bug report.
mIRC has supported a /msg -s switch since v3.9 but only if it was exactly "-s". Awful design decision. But someone requested -s support and I am guessing we were worried that adding -switch support would result in the issue currently being reported.
The /msg routine is also used by /query to send a message and/or open a window. /query supports combinations of switches -nxmd. Prior to 7.52, it supported only -n.
When I was updating /msg and /query in v7.52, I must have decided to merge support for these switches together for consistency and predictability, which has caused the current issue.
I will revert these changes in the next version. This means that /msg will go back to working how it did before: -s will still be a switch but any other "-" prefixed word will be treated as a nickname. This also means that /query will no longer support -s.
In retrospect, I should have made all /commands treat "-" as a switch if it was the first parameter, even if they did not support any switches, to make behaviour predictable. As this was not done, extending non-switch commands with new switches/features without breaking scripts just leads to issues like this one.
