Ah… this issue.
It all started here: Encoded slash in URL · Issue #643 · caddyserver/caddy · GitHub
In that thread, follow the linked PRs for more discussion, particularly this one: Encoded slash in URL · Issue #1582 · caddyserver/caddy · GitHub
The good news is, I think in v2 we could handle this much better. I’d be very interested to know how this works in v2 currently, and since we’re still in beta we can find ways to handle it better if we want/need to change it.
Can you please give it a try on Caddy 2 and see how it goes?