Hi, just after my BC surgery my daughter had a tiny breast lump that was mucinous that needed biopsy and then removal, and we were not sure if she was about to go through the whole breast cancer trip along side me. Luckily it turned out to be not cancer, but at the time we did a lot of reading about it.
It is a very unusual form of cancer, and hard to get information about, but I remember they talked about pure mucinous type and mixed mucinous type. The pure form is pure mucinos tissue, is slow growing and unlikely to metastasise and has very good results. But if it is "mixed" they have to treat mucinous cancer cells plus whatever the other sort, often IDC that are mixed with it.
Also my daughter found research that showed that this sort of cancer is harder to spot than many. They are often multiple but the surgeons don't discover that until surgery, and radiographers are more likely to miss metastases on the initial scan for them. The researchers also said oncologists often undertreat them when it comes to chemotherapy,.
The bone scan and body scan we have before surgery to check for metastases only show lesions over 1cm across, so maybe yours was already there but too small to show up on the scan, and did not get hit hard enough with the original chemo. If they wallop it hard this time, it is highly likely they will clear it out for once and for all. Which would mean you have an excellent chance of having several more decades of life ahead of you.
best wishes