Honestly, under the circumstances I can understand why they may not have gone out of their way to get you a leaving gift, especially at this time of year when it's busy enough as it is.
If you have been off for months then your colleagues may be unaware of your actual leaving - have you stayed in contact with your team? Are you friends outside work with any of them? I work in a supermarket and over the last couple of years we have had a few people who went from long term sick to either retiring or being managed out due to their medical issue. The only reason I was aware of the colleagues who were managed out is because, as a union rep, I was involved in the process whereas the other colleagues were not kept up to date with the process and when someone went from "off sick" to "no longer working for us". The bosses don't make big announcements that "X has been managed out" or "Y has retired", that is left for the colleagues themselves to make people aware of (or not) as they choose.
They also may feel like, if they haven't seen you in months, you "left" months ago and they already processed your departure when you went off on bereavement so it hasn't occurred to them to go out of their way to also get you a leaving card now when you haven't been in since they sent a card/flowers for your bereavement.
Finally, if you haven't been in for months then your team may have had significant changes over the last few months that you may not be aware of. Especially in the NHS, people have been redeployed to different teams/departments/shifts etc, so your existing team may have had a lot of coming and going, the current team may be redeployed and/or have different team members, they may have been given more work to do etc and they haven't got someone to coordinate a leaving card or gift for someone who has been off for months.
It's shit and it sucks to effectively be forgotten about so quickly but in the nicest possible way, people leaving are not usually top priority, especially if you haven't been in for months.