Cause of rain ingress.

Has the roof been replaced correctly ie all lead work correct, any brickwork in need of repointing or rendering? Any chimney pots not capped off or got chimney cowls on?

If rainwater is seeping into your upstairs walls despite a recent roof replacement, start by inspecting the roof, flashing, gutters, and downspouts for damage or blockages. Check for leaks around windows and doors, and assess the condition of your siding and masonry. If the issue persists, consult a professional for a thorough inspection. If your roof replacement is under warranty, contact the roofing company for assistance. Addressing this issue promptly is crucial to prevent further damage.

As previously stated it could be a number of things causeing the ingress, something simple like a blocked skew or tile not sitting properly, best bet is to get someone out to give the roof a look over,

This could be a dampness issue due to lack of ventilation in the new roof install. When installing a new roof we would use a breathable membrane, install tile vents and a dry ventilated ridge system, this creates a continuous air flow in the loft space.