SDK and version

SDK : iOS
Version: 10.43.0
Core: 13.22.0

Observations

• How frequent do the crash occur?
  32 users and counting (~0.8% of the users of now)
• Does it happen in production or during dev/test?
  Production
• Can the crash be reproduced by you?
  No :/
• Can you provide instructions for how we can reproduce it?
  We are observing sectioned results on a custom computed key. Maybe it can help to know that this key is not managed by realm.

Crash log / stacktrace

EXC_BAD_ACCESS
Exception 1, Code 1, Subcode 88 >
KERN_INVALID_ADDRESS at 0x58.

Demangled symbols shows the crash at the following line:

sectioned_results.cpp:124:

 for (size_t i = m_change.modifications.size(); i > 0; --i) {
            auto& indexes_old = m_change.modifications[i - 1];
            auto key = m_sectioned_results.m_previous_index_to_key[i - 1]; // <- Offending line
            auto it = m_sectioned_results.m_current_key_to_index.find(key); 
            if (it == m_sectioned_results.m_current_key_to_index.end()) {
                // Section was removed due to all of the rows being moved to
                // other sections. No need to report the individual rows as deleted.
                indexes_old.clear();

Steps & Code to Reproduce

We can't reproduce the issue but our code is open source here: https://github.com/Infomaniak/ios-kMail/pull/1037/files#diff-c43bb011c6baa01d7daa6630b1cb91c05ed05a927e7529ab3895b63896083ba3R47

I posted an issue on the swift repo but since the crash happens in core I think it's better to post the issue is here.
(https://github.com/realm/realm-core/issues/7321)