Late b1 trigger by UE after new LTE RRC connection

I will share the exact case:

UE is on LTE B2 (1900 Band) and NR is n5 (850 Band).

-UE makes new RRC connection on LTE

-UE gets b1 thresh in RrcReconfig to add NR leg and b1 thresh is pretty relaxed.

-UE starts downloading the data right away after making LTE RRC

-But There is no eventb1 triggered by UE right after receiving b1 from eNB. Even though NR coverage/quality is very good and way under b1 thresh

-UE shares b1 measurements very late randomly after 10 seconds or 20s and sometimes after 40-45 seconds.

-Since its new LTE RRC conn so no penalty timer running also there is no data thresh.

-One strange scenario is, in RRC conn mode when UE does LTE HO while NR leg is also present. It sends b1 MR right after latching to target LTE cell.

-So what could the issue that UE is taking too long to send b1 MR after new LTE RRC conn? (Vendor:Ericsson)

If enb and gnb are not synchronized, ue may miss SSB and do not send B1

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But at the time of LTE HO, UE sends b1 ASAP after latching to target LTE cell consider NR leg was present on source LTE cell.

(because LTE HO breaks the NR leg and UE has to send b1 again to add NR leg on target LTE cell).

Are lte source and target cells on same bbu?