Interesting find, as that does appear to confirm that the other ACX platforms now do support 100Mb optics, but not the 5000 series.
https://www.juniper.net/documentation/en_US/junos/information-products/topic-collections/release-notes/17.3/topic-120780.html#rn-junos-acx-new-and-changed-features
Support for 100 MB Optics (ACX500, ACX1000, ACX1100, ACX2100, ACX2200, ACX4000)—Starting in Junos OS Release 17.3R1, ACX Series Universal Access Routers (ACX500, ACX1000, ACX1100, ACX2100, ACX2200, ACX4000) support 100 MB Ethernet optics.
Bummer.
Some of our services are handed off to us on 100Mb copper.
I guess we'll have to put an intermediary EX switch between the ACX5048 and the network provider's NTE.