The exception to this would be e.g. I intend to raise $150k, i get this amount of money in the bank and then more investors want to participate. I can bring them in but I could argue for a higher valuation since they're coming in once I have money in the bank i.e. at less risk than the investors who initially put in $150k and therefore deserve the lower valuation.
Other exceptions might be where an investor brings particular value that warrants a lower price e.g. you're working on payments and Max Levchin wants to invest
Convertible notes are loans that can be converted into equity if some predetermined event takes place (usually you raise another round of money or you get a buy out offer.)
You're not selling X shares at Y price. You're issuing a loan that may get converted into X shares. Every investor who gets these convertible notes usually will end up getting the same valuation in the future if the note converts to equity.
So with rolling round, you're just taking on debt. And you can take as much debt as you want to (unless you've added a clause in any of the notes that determines that you can't go over a certain amount of debt.)
that's not true, most of the convertible notes we're seeing include a valuation cap in them i.e. the investor never converts their debt at a valuation higher than the cap. the cap addresses the main criticism of convertible notes, namely they misalign the incentives of the founders (who want a high valuation for their next funding event) and investors (who want a lower valuation for the next round so they get more stock).