What could cause QFX5110 interfaces to only come up when configured as mge but not ge or xe?

These are the interface naming prefixes used in networking operating systems:

ge means Gigabit Ethernet, and xe means 10-Gigabit Ethernet on interfaces.

While mge refers to multirate Gigabit Ethernet interfaces that can support multiple speeds depending on hardware and configuration.

Based on the QFX5110 Port Panels hardware guide, the port panel of the QFX5110-48S supports 48 logical 10-GbE ports when operating as a standalone switch.

These data ports (0 through 47) support either 1-Gbps or 10-Gbps SFP+ transceivers. You can also use SFP+ DAC cables and 10-Gbps active optical cables (AOC) in any access port. Starting in Junos OS Release 18.3R1, the 10-GbE ports also support 100 Mbps.

The remaining 4 QSFP28 ports (48 through 51) support speeds of 40 GbE or 100 GbE. Each port can be configured as an independent 100-GbE port or as an independent 40-GbE port. 

Please review this documentation below: https://www.juniper.net/documentation/us/en/hardware/qfx5110/topics/topic-map/qfx5110-system-overview.html

I also looked for the 740-083295 part number for SFP+; however does not seem to be supported for this platform: https://apps.juniper.net/hct/product/QFX5110/100001?isCommonSku=No&isModelEol=No

Important: We highly suggest using an SFP which has been tested by Juniper like the one included in the list above.

In this particular case, if the port came up only as mge, that likely means the hardware/platform and transceiver combination (740-083295) in used expects the mge interface family to properly work as 100Mbps.