I am absolutely not an expert on freesias but that looks horribly like virus infection to me. I'd burn the lot, I'm afraid. Freesia sneak virus

It reminds me of my gladiolus foliage when I was a new gardener. I was told to destroy them. Instead I experiented after the foliage aged. Removed it. Dug up the corms and soaked them a couple of hours in a malathion solution (only chemical killer I had). Dried and replanted a month later. Had clean foliage the following year. Not saying this is your solution ...... but if they're going to be goners and you have malathion ....... It may be a virus and that can't be treated.

I googled freesia thrip and found this.

"In many thrips species, by the time their damage is observed, such as
after buds open, the thrips may no longer be present. Some abiotic
disorders, pathogens, and certain other invertebrates can cause damage
resembling that of thrips. For example, various true bugs and mites also stipple foliage;"

Thank you floral_uk and irisgal_z9! This is my first time attempting to grow Freesias, and what can I say, failures are the pillars to success I guess? I really don't want to destroy them all though, so I plan to dig them up today and replant in fresh potting soil. I am not sure whether I should replant the "infected" ones along with the healthy ones, or just dispose the former off. Any advice?

Yep, I thought virus too. Not usually any cure although seeds can remain free of virus...so if you do manage to get a bloom, it might be worth saving any seed. I have certainly kept lilies going by saving the seed but it is a slow process and only worthwhile for expensive stock.

At the very least, keep the infected stock separate from healthy corms. The virus is soil borne but even repotting is no guarantee that infection will not cross over into fresh stock.

Very interesting and helpful! I have seen a freesia virus before. I will keep a lookout for it on mine!