IPv6 host does not respond to Neighbour Solicitation

The notation in that case is to encode the IPv6 IP number in square brackets:

http://[2001:db8:1f70::999:de8:7648:6e8]:100/

That's RFC 3986, section 3.2.2: Host

A host identified by an Internet Protocol literal address, version 6 [RFC3513] or later, is distinguished by enclosing the IP literal within square brackets ("[" and "]"). This is the only place where square bracket characters are allowed in the URI syntax. In anticipation of future, as-yet-undefined IP literal address formats, an implementation may use an optional version flag to indicate such a format explicitly rather than rely on heuristic determination.

It sounds like you've already done captures. If you've verified your address resolution requests are being sent, but you're not receiving replies, that sounds like either their router isn't responding or a layer 2 device in the path is dropping them - for whatever reason, be it bug or configuration. If they're willing to provide configurations, you might also ask about the actual physical interface configuration here (ge1/48) and any switch ports the VLAN is trunked through on their network.

If you're sure it's not your end (Are you direct to the provider from your router? Attempted other hardware?), then I'd say push for debugs at the Cisco and/or span/port mirroring on their ingress switch port to capture and confirm if they're seeing your requests inbound. If they confirm they are and they aren't replying, they're somewhat obligated to determine why from there.