Thank you @namusyaka, I'm happy to merge this but will wait for @mwpastore's seal of approval ;)

Any progress on this?

@mwpastore Could you check about this?

Hi @namusyaka, I pinged you on the Slack about this, but I guess you didn't see it. I'm getting hung up on this because I don't really understand the solution. Can you help me understand the why and how? That being said, my review is superfluous if it solves the problem.

@mwpastore @zzak Sorry for the late reply.
I will explain the issue.

The sinatra namespace is prefixed to verb and filter methods (see https://github.com/sinatra/sinatra/blob/master/sinatra-contrib/lib/sinatra/namespace.rb#L232) by using original methods of indivisual namespaces.
The prefix rule is defined in this line, but the filter is a special one using the constant regexp.
Root of your issue is here.

Please take a look at these lines.
The before filter is registered with the regexp and the prefix for the namespace.
prefixed_path will make the concat pattern which is something like:

require 'mustermann'

# These vars are prefix of namespace.
foo     = Mustermann.new('/foo')
foo_bar = Mustermann.new('/foo-bar')

# current pattern
regexp = Mustermann.new(/.*/)

p (foo_bar + regexp) === '/foo-bar' #=> true
p (foo + regexp) === '/foo-bar'     #=> true (but expected to return false)

##############################################

# my patch
regexp = Mustermann.new(%r{(?:/.*)?})

p (foo_bar + regexp) === '/foo-bar' #=> true
p (foo + regexp) === '/foo-bar'     #=> false

Under the above principle, even filter which does not expect to operate originally has been executed on the current pattern.
Thus, your issue has been occured by calling twice extend method.

So my patch makes it avoid the unexpected matching with filter. I guess this is reasonable. Thoughts?

Because this patch affects namespace, which was changed in 2.0 to support Mustermann, we are fine to merge it.

Thank you @namusyaka for your patch and explanation!

Thank you for the patch and the explanation, @namusyaka, and thank you both for your patience! 🙇