That's normal issue for 2.5GbE at GPON, we actively trying to solve speed drop when using 2.5G ONU.
This happen because GPON not meant for 2.5G, when packet collision (drop) happen because Ethernet (Symmetrical) to GPON (Asymmetrical):

By default, D-Link kind of wrong use Flow Control, when packet wait filled the RAM and speed dropped,
computer also doesn't know the gateway only 1Gbps Upload,
Who under Nokia OLT with correct OMCI (can bridge) and DSCP (proper QoS), no issue.
but, not all under Nokia OLT, mostly Fiberhome and some cheap brand OLT that not meant for 2Gbps plan.
TM should use XGSPON for 2Gbps
Also I notice D-Link DPN-FX3060V A1 and B1 has three PON Mode:

0 = GPON (2488M/1244M)
1 = XGPON (10G/2.5G)
2 = XGSPON (10G/10G)
Could be TM future is 10Gbps due to TM no longer allow to expand, the fiber budget has given to Allo/Maxis
Another theory why B1 hardware cannot bridge because of 2.5G Switch Chip connected to SoC are not MePoint 0x101 (ME 171)

The MePoint might wrong and Firmware set to wrong exit:

There are two possible fix:
1. D-Link need a patch or hack ME 171
2. OLT need properly set correct ME 171 (because different place has different ME 171 even same subscriber)
my 2Gbps account was activated using PON Stick, and I ask to reactivate again using latest my modded PON Firmware and I can set D-Link ONR B1 to bridge mode, but WebGUI is killed by Nokia OLT (ME 171 tell to disable):

That's it hold my beer im calling tm to hire you.