spotx: adding ignoreCacheKey note (#2874)

dev-docs/bidders/spotx.md

@@ -12,8 +12,14 @@ usp_supported: true
 safeframes_ok: false
 pbjs: true
 gvl_id: 165
+pbjs_version_notes: avoid 4.31-4.35
 ---
 
+### Note:
+
+There was an issue in the SpotX adapter in PBJS 4.31 that assumed all publishers had already set the [`ignoreVideoCacheKey`](/dev-docs/publisher-api-reference.html#setConfig-vast-cache).
+Publishers using SpotX that haven't yet updated their ad server line items and turned on the ignoreVideoCacheKey option should avoid Prebid.js 4.31 - 4.35.
+
 ### Bid Params
 
 {: .table .table-bordered .table-striped }