Rollup fields only work in conjunction with linked record fields.

Do the numbers from JIRA get sent as the comma separated list into a single field? If so, Airtable treats it as text. You can convert a text string that looks like a number into a number using VALUE() but it only works for one number.

Are there always four numbers, or could there sometimes be more or less? Is there a maximum number of numbers?

Can you post screen shots of all the relevant fields, including the field populated by JIRA, any linked record fields, and your rollup/formula field?

The fields are being pulled through from JIRA as single values but pulled into a comma separated list as a result of grouping them in Airtable.
There could be any number of these numbers, not just four.

The values are being pulled in like this. I use a formula to pull out the number from 20220526-PP-R151-…
to look like: 151 as I only care about this part.

I then group various JIRA ID together in another table which creates a view like:
FeatureA - 141,142,143 (etc)

I need to then just need to print the highest of these values.

Thank you for the screen captures and the description of how you are getting the values. Since your numbers are left aligned, I bet your numbers are actually text strings that look like numbers.

In your formula field that pulls out the number, wrap the formula in VALUE() to convert the text string into an actual number. The number should now be right aligned.

Then in your other table, use a rollup formula with the MAX(values) aggregation formula.