When it is a discriminatory price auction bid, people will not anyhow enter a bid. If the bidder puts in 2% and wins the bid, they will get it at 2% despite the COY being much higher. So everyone along the chain until the cut-off rate pays whatever they put in their bid rather than following the highest COY. As per the below report, there is still a whole list of countries using discriminatory auctions, so it is not something uncommon. Regardless of retail or institutional investors, there will still be bidders putting in low bids, MAS bill auction also reflects a similar pattern because the average yield can also be quite far from COY.
https://www.econstor.eu/bitstream/10419/141995/1/796715769.pdf
If retail bidding drags down the yield, especially under uniform price auction, then it shows that the market lacks the required liquidity which may be true. In general, the govt doesn't need any debt (other than the longer-term infrastructure program) to fund budget spending which is quite different from many others, the main purpose of debt issuance is to provide liquidity and benchmarking for the debt market. In a high-interest rate environment, the problem becomes whether the income generated from issuing more debt can offset the interest cost.
Yes, you are right. They moved to uniform auction sometime back in 2007. But their liquidity is huge, so it probably has little impact. For smaller countries, any sudden big inflow of funds can move the yield by a lot. I suspect that nowadays, the T-bill markets are driven by retail investors, those money market funds which I am looking at, almost all are buying into MAS bills which have a much higher yield compared to the 6-month T-bills. Back in early 2022, the spread between 3 months MAS bills and 6 months T-bills was around 5 to 10 bps, now it is much wider at 30bps and that is already taking into consideration that the issue size already increased from 4bil++ to 6bil++.
I think our market lacks liquidity depth despite the high volume of secondary transactions going on. The 1/3/6 months SORA are around 3.5%/3.6%/3.7%, but 3 and 12-week MAS bills are still being auctioned at more than 3.9%.