I think most people simply do not understand that what someone else does to you can elicit suicidality, even if it's not their fault nor their intention to do so. Nor is it their obligation to have their decision making swayed by it, unless they genuinely care about the other person and would have chosen to stay regardless of the other party's suicidality. Sometimes, shit happens and people's fleeting presence and absence can deeply wound you, even if we have very little control over the thoughts and feelings of others and have to let them choose their own path. It isn't immoral by any means.. Just simply an unfortunate reality of life.
However, it is far easier for people to cry manipulation or throw out accusations of attention seeking than to recognize that human interactions, emotions, and behaviours are a complex, multifacted engima that can't be boiled down and reduced to the basic motivations. Sometimes people can do actions which really hurt, but there is no moral high ground to be had, it's just unfortunate and tragic- for example, a break up.
While there likely are a few people out there who falsely claim to be suicidal for malicious reasons, those people clearly have some other underlying issue at play that isn't understood or able to be managed by modern medicine. But I'd say this number is so small and insignificant compared to the number of people who are genuinely suicidal and have no ill-intentions in disclosing it to someone.
I think the accusations of manipulation only stem from societal norms about suicide, wherein other people are held responsible for prevention at all costs. If this weren't drilled into everyone's head as a moral principle, that they must forcibly stop suicide at all costs and if they don't, it makes them an awful person who failed their loved one and potentially committed an immoral act, I don't think there would be such an outcry about suicidal ideation being manipulative.
If others weren't forced to have the obligation and responsibility of prevention at all costs, and to shoulder the burden of guilt if they're unable to influence the suicidal party's decision, I don't think these accusations of I'll-intent would be so prominent.
I am so sorry for what you're going through with your partner. In a more compassionate world, you'd be able to discuss your honest thoughts and feelings about this matter, and it is so awful that you have to feel so alone and lack the space to have an honest conversation about this topic without being accused of malice.