That’s a symptom, not the root cause. This topic started because wiki:symbol is dangerous to implement naïvely. wiki:symbol exists because, despite our best efforts, osmc:symbol will always be incomplete. Hopefully the mitigations I suggested will be helpful, but there are a lot of other problems with the idea of loading arbitrary tagged URLs or image names. This may explain why only one renderer has previously attempted to consume wiki:symbol – the renderer that serves as the reference implementation of osmc:symbol. What other key do you know of that replaces the “Any tag you like” principle with the authority of one person?
If you need a frequently used symbol added, contact Nop with suggestions. Don’t invent a new tag, simply use text/textcolor or refrain from using osmc:symbol altogether.
My point is that ideally the symbol would be derived from ref and a more meaningful network-like key. Consider the fact that most of the routes in Croatia have the same symbol design: is this a mere coincidence, or is it a consequence of these routes belonging to the same network? If a particular renderer is misinterpreting osmc:symbol, then by all means, the bug should be fixed. But please note that, if we keep putting effort into building out the osmc:symbol language without solving the problem more generally, then recreational route symbols will remain limited to a small club of renderers.