Automated triage: Not a bug
Thank you for reporting this issue. Since this was reported, XenForo 2.3.13 added a limit on how deeply BB code tags can be nested, inline spoilers included. Anything past 20 levels is shown as plain text. If a page with nested spoilers still fails on mobile on the current version, please open a new report with the post content and device details.
This is an automated assessment. A member of staff may review and update it.