As i did say i don't have openreach full fibre myself and have Orange FTTP and SFR Cable services (much better than VM and a lot cheaper than Openreach wholesale costs).
My acquaintance did not fully brief me on what the issue at Openreach's end was as they were unable to find out, Openreach never told them the full details of what caused the issue.
I can tell you that the openreach engineers kept saying they had turned up to fix the problem however the CCTV footage (now deleted due to storage restraints) proved otherwise.
Apparently the person said that at random times there would be "disconnections" where the connection was intermittent which I personally thought could either be the ONT, Router (before knowing it was excluded), some questionable splicing that kept being knocked or potentially some type of Fibre Optic Switch (I'm not sure what Openreach use themself so cannot comment on this). They had tested directly at the ONT on various devices to ensure that it was not a router causing issues which was confirmed. I believe openreach checked the fibre and said it was working fine (which being intermittent it would be) however later on the customer called back the ISP (ZEN) which contacted Openreach again where they missed more appointments again. However they were lucky and had the same engineer (like me with the same VM engineer 15 times this year, 10 In jan) the engineer changed the ONT so assumed the problem would go away however later on it came back so another phone call to ZEN was made where openreach sent another engineer which then said it's not this end it must be further up and said that the aggregation node was not an issue and said it was the headend exchange.
This then leads to the exchange swapping out some network hardware which then seems to solve the issue at the end of that 30 days of fun.
This was easily over a month of complaining (so not as bad as me with Virgin Media for over a year now)
I wish i could tell you more but that is what was said to me by that person so I cannot help more unfortunately.
Let me know if that gives you some kind of idea but I can only guess anything i was not told.