Shimano Di2 Bleed Screw problem

which spec are you looking at? source/link? I cannot find any reference to that part number online. the correct part number is Y-0C6-98030, which is two characters away from your non-existent part number

https://si.shimano.com/api/publish/storage/pdf/en/ev/ST-R8070/EV-ST-R8070-4248B.pdf

this is for current D-A/Ultegra levers

https://www.sjscycles.co.uk/gear-spares/shimano-duraace-str9120-bleed-screw-oring-y0c698030/

there is also https://www.sjscycles.co.uk/gear-spares/shimano-105-str7020-bleed-screw-oring-y0f398030/

which is Y-0F3-98030 for current 105 & Tiagra levers

the same -0C6- and 0F3 are used respectively by RX800 ('Ultegra' level) and RX600/400 (105/Tiagra level) series

Older levers use

Y-8V3-98010 (R785/RS685/RS505/RS405/U5060/U5010)

https://www.sjscycles.co.uk/brake-levers/shimano-xtr-blm988-bleed-screw-oring-m5-x-47mm-y8v398010/

Since there appears to be only three possible designs, one for all older road designs (2.5mm hex key), and a '105/Tiagra' (2.5mm hex key) and a Dura-Ace/Ultegra screw (2mm hex key) for newer designs, and you have given a non-existent part number, then it stands to reason that you have ordered the wrong part.

Is your existing screw a 2.5mm or a 2mm head? And how about the new one?

edit:

with the addition of the 'My bike is is 2018 Cannondale Synapse Di2.' to the OP my guess is that the lever as revised from ST-R785 DI2 (2017 MY) to ST-R8020 (2018 MY). Probably you have ST-R785. I would not refer to 'model years' but look at the actual components fitted, as they change during the year often. the shifter model number will be marked on the body. In this case you need Y-8V3-98010